1. Field of the Invention
The present invention relates to a method of measuring distance using an ultrasonic wave sensor and an apparatus therefor, more specifically, relates to a technique which enables measurement of a long distance by an ultrasonic-wave distance measuring method and an apparatus of the separate transmission and reception type reflection system which measure a distance using two water-proof ultrasonic-wave sensors.
2. Description of the Related Art
Distance measuring methods using ultrasonic-wave sensors are known.
These systems for measuring distance using ultrasonic-wave sensors may be roughly divided into two groups: distance measuring apparatuses of a single-sensor system which use a single ultrasonic-wave sensor for both transmission and reception and distance measuring apparatuses of the separate transmission and reception type reflection system which independently use an ultrasonic-wave transmission sensor and an ultrasonic-wave reception sensor.
The distance measuring apparatuses of the single-sensor system are advantageous because the price of the ultrasonic-wave sensor becomes low since only one ultrasonic-wave sensor is used. However, since the ultrasonic-wave sensor is used for both transmission and reception, the circuit has to be switched at the transmission timings and reception timings and it is difficult to measure short distances in some cases.
An ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system is an ultrasonic-wave distance measuring apparatus of a dual-sensor system provided with an independent ultrasonic-wave transmission sensor and ultrasonic-wave reception sensor and overcomes the above disadvantage of the distance measuring apparatus of a single-sensor system.
The present invention relates to an ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system.
FIG. 1 is a view of the configuration of an ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system.
This ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system includes a transmission side having an ultrasonic-wave transmitter 1, an ultrasonic-wave transmission sensor 3, and a transmission horn antenna 5 and a reception side having an ultrasonic-wave receiver 11, an ultrasonic-wave reception sensor 13, and a reception horn antenna 15.
In this ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system, the transmission side and the reception side are installed in a certain apparatus, for example, a vehicle, and can be used for measuring a distance from the vehicle to a distance measured object 7. As an example of measuring distance, the detection of obstacles for vehicles can be mentioned.
The method of measuring a distance by the ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system shown in FIG. 1 will be described next.
The ultrasonic-wave transmitter 1 energizes the ultrasonic-wave transmission sensor 3 to make the ultrasonic-wave transmission sensor 3 generate an ultrasonic wave. The ultrasonic wave from the ultrasonic-wave transmission sensor 3 is directed to the distance measured object 7 via the transmission horn antenna 5 and strikes the distance measured object 7.
The ultrasonic wave reflected on the distance measured object 7 returns to the ultrasonic-wave reception sensor 13 via the reception horn antenna 15. The ultrasonic-wave reception sensor 13 outputs an electric signal in response to the level of the incident ultrasonic wave. The ultrasonic-wave receiver 11 measures the time from emission of the ultrasonic wave by the ultrasonic-wave transmission sensor 3 to reception of the same by the ultrasonic-wave reception sensor 13 and calculates the distance between the distance measured object 7 and the ultrasonic-wave transmission sensor 3 from the measured time.
In the ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system shown in FIG. 1, when used for a vehicle or the like, ultrasonic-wave sensors of a waterproof type are often used as the ultrasonic-wave transmission sensor 3 and the ultrasonic-wave reception sensor 13 to make them resistant to the strong rain outdoors.
Turning now to the disadvantage to be overcome by the invention, because of the cover provided over an ultrasonic-wave sensor of the water-proof type, the total gain is reduced. Therefore, when using ultrasonic-wave sensors of a water-proof type as the ultrasonic-wave transmission sensor 3 and the ultrasonic-wave reception sensor 13, the range of measurement of distance becomes shorter. For example, when using ultrasonic-wave sensors of the water-proof type, it is normally only possible to stably measure a distance of 4 to 5 meters.
However, when using an ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system as a proximity sensor of a vehicle, the measurement of a longer distance is desired. It is therefore desired to improve the ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system using ultrasonic-wave sensors of water-proof types.
One method to make the measurable distance longer is to raise the voltage for exciting the transmission ultrasonic-wave sensor 3 within the range of the allowable applied voltage. However, if the voltage for energizing the ultrasonic-wave transmission sensor 3 is raised, the ultrasonic-wave transmission sensor 3 will vibrate more and the vibration will continue for a longer time. As a result, the ultrasonic-wave reception sensor 13 installed near to the ultrasonic-wave transmission sensor 3 will generate an electric signal in response to the vibration of the ultrasonic-wave transmission sensor 3 which will make accurate distance measurement difficult. Especially, an accurate measurement of short distances will become difficult.
As a countermeasure for this, it is necessary to separate the members for mounting the ultrasonic-wave transmission sensor 3 and the ultrasonic-wave reception sensor 13 and to take steps to prevent the vibration at the plate mounting the ultrasonic-wave transmission sensor 3 and the plate mounting the ultrasonic-wave reception sensor 13. However, these anti-vibration countermeasures are complicated, they enlarge the size of the apparatus, and make mounting conditions in the vehicle tougher. Furthermore, the price of the ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system becomes much higher and installation in a vehicle becomes difficult in terms of cost. Accordingly, there are limits to the method of raising the voltage for energizing the ultrasonic-wave transmission sensor 3.
Normally, the ultrasonic-wave receiver 11 is provided with an automatic gain control (AGC) circuit for automatic gain control of the received ultrasonic-wave signal. However, the automatic gain control has the following disadvantages described with reference to FIG. 2. FIG. 2 is a graph showing the automatic gain control.
Automatic gain control is, generally speaking, a correlation response of the level of an input signal. Accordingly, the apparatus operates at the same total gain for different distances being measured, so operates at the total gain for short distances as well. In FIG. 2, if the relative gain at the point b at the input level of the point a is made zero (reference level), the relative gain becomes maximum to the maximum differential value of the ultrasonic-wave transmission sensor 3 when the input level is low. As a result, in the case of a short distance measurement, the ultrasonic-wave reception sensor 13 is easily electrically influenced by the mechanical vibration of the ultrasonic-wave transmission sensor 3. To prevent this, countermeasures for the vibration of the ultrasonic-wave transmission sensor 3 are necessary, however, such countermeasures for vibration have the disadvantages described above.
Contrary to the above discussion, when adopting a system where the response falls as the level of the input signal received in the ultrasonic-wave reception sensor 13 becomes larger, the sensitivity at the time of measuring a long distance falls tremendously, so such an ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system becomes unsuitable for long distance measurement.
The sensitivity to the reflected ultrasonic-wave from the distance measured object 7 differs when the distance measured object 7 is, for example, metal, concrete, or a human body. Therefore, processing for measuring distance which takes into account the type of the distance measured object 7 is necessary for stable precise distance measurement. However, such processing has not been carried out.
When using the ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system shown in FIG. 1 for an apparatus for detecting obstacles for vehicles, when the speed of the vehicle becomes more than a certain value, the noise of the reception antenna of the vehicle passing against the air, the noise of the tires running over the road surface, so forth generate ultrasonic waves. It was found that these were detected by the ultrasonic-wave reception sensor 13 and resulted in erroneous operation in the distance measurement.
While the above considered the case of use of the ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system to vehicles as an example, similar disadvantages as the above are encountered not only when mounting an ultrasonic-wave distance measuring apparatus of the separate transmission and reception type reflection system to a vehicle, but other cases as well.